RE: AW: [Xen-devel] PCI bus emulation?

["Neugebauer, Rolf" <rolf.neugebauer@xxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Mark Williamson
> Sent: 04 August 2005 16:42
> To: Stefan Berger
> Cc: Retzki, Sascha [Xplain]; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: AW: [Xen-devel] PCI bus emulation?
> 
> > It shuld be possible for domain 0 to read all devices from the bus and
> > build its list. Then domain 0 could take the to-be-hidden devices out of
> > that list
> 
> Yep.  Do you think it's possible to simply chop these devices out of the
> list
> after initialisation but before passing up to the generic PCI layer?
> That'd
> be excellent.

A while back I had a look at this and it didn't look as if it would be very 
straight forward by just using arch specific code but probably doable using the 
hooks provided.

> 
> > and maybe 'program' the virtualized PCI bus of a driver domain
> > to show these devices (through sending a message to the HV about what to
> > present).
> 
> I'd be inclined to have this purely implemented at kernel level using
> interdomain comms, rather than having Xen know anything about it.

I'm not entirely convinced that this is the right way to do. Once the devices 
are set up by dom0 xen wouldn't have to do that much and intercepting pci 
config read/writes in xen using the instruction emulator might be better and 
create less dependencies on Dom0.

The code in Xen 2.x dealing with PCI config reads/writes by device driver 
domains is actually quite small. Extending this to use the instruction decoder 
should be quite straight forward and less complex than doing a PCI config space 
frontend/backend type thing. And it would make changes to guests smaller than 
they already are in 2.x

rolf

> > A driver domain would still have to be co-operative in that
> > sense that it first starts out with a low privilege level (by default a
> > domain would be created that way) to run into the exception handler and
> > request to have its privilege level raised once it wants to access the
> raw
> > hardware. Setting the individual privilege bits for a driver domain
> might
> > be another possibility, but you'd  need to know the list of ioports for
> > each possible device.
> 
> Right now we do just that: domains never get access to the PCI controller,
> so
> config space accesses must always be mediated by Xen.  Domains are
> assigned
> direct access to the devices they control using IO port bits (although I'm
> not entirely happy with that, since it allows applications in the domain
> access to the device - fine for a driver domain but not good for
> partitioning
> the hardware) and controlled mapping of IO memory.
> 
> This works now because Xen can read the required IO port / memory ranges
> out
> of PCI config space and enable them as appropriate.  To make this work
> with
> the unstable tree, we'd need to have dom0's kernel somehow get this
> information out...
> 
> > > Using emulation to implement b) would have the advantage that the
> >
> > current
> >
> > > probing code shouldn't need changing.  OTOH, the overall system may be
> > > simpler if the guest detects it's not allowed to access the
> >
> > hardwaredirectly
> >
> > > and explicitly talks to dom0 e.g. using Xenstore to probe its devices.
> >
> > I also think that it would have the advantage of being able to build a
> > user domain independently of whether it will become a driver domain or
> > not. All the difference would be in the virtualized PCI bus presenting
> > devices or being empty.
> 
> I definitely think we should retain the ability to build one kernel that
> works
> in all possible domains.  The startinfo does contain flags that tell the
> domain if it's dom0 (and therefore controls the PCI bus) or not.
> 
> Cheers,
> Mark
> 
> >  Stefan
> >
> > > Cheers,
> > > Mark
> > >
> > > > I don't get the purpose. What is wrong with hiding certain PCI
> devices
> >
> > from
> >
> > > > DomO and thus make them available in domU?
> > > >
> > > > Btw, you are sure all OSes "find an empty bus"?
> > > >
> > > >
> > > > -----Ursprüngliche Nachricht-----
> > > > Von: Stefan Berger [mailto:stefanb@xxxxxxxxxx]
> > > > Gesendet: Mittwoch, 3. August 2005 22:47
> > > > An: xen-devel@xxxxxxxxxxxxxxxxxxx
> > > > Betreff: [Xen-devel] PCI bus emulation?
> > > >
> > > > Hello!
> > > >
> > > >   I have seen that the Qemu code contains some nice code for PCI bus
> > > > emulation. I wonder whether this code could be reused in XEN to fake
> a
> >
> > PCI
> >
> > > > bus by running the PCI emulation code in an exception handler in the
> > > > hypervisor and setting a user domain's IO privilege level
> >
> > appropriately to
> >
> > > > have all inb/outb's intercepted. This could have potential benefits
> on
> >
> > the
> >
> > > > build process of user domains which could include the PCI code when
> >
> > built,
> >
> > > > but that code when probing the PCI bus would only find an empty bus
> >
> > and
> >
> > > > the probing of the drivers in the kernel would not start. Maybe this
> >
> > code
> >
> > > > could also be used to support driver domains. Is this a good idea?
> :-)
> > > >
> > > >   Stefan
> > > >
> > > >
> > > >
> > > >
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> > > >
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> 
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